RISCWORLD 5/RISCWORLD_VOL5.iso/SOFTWARE/Issue5/PD/FLOWER/TINCT009/!Help

Text File | 2004-11-02 | 10.6 KB | 252 lines

Tinct
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This module provides the necessary functionality to display alpha-blended
sprites both scaled and otherwise. It also provides functions for dithering,
error diffusion and performing bi-linear filtering to improve their appearance.


Technical information
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To ensure future compatibility, this module does not patch the OS in any way
and works in a totally legal way. It also does not write to itself in any
way, so is suitable for running from ROM.
 Redirection to sprites is supported, although due to the overheads involved
with caching the colour translation tables it is not recommended that this is
done frequently. There are some exceptions to this, however, as redirecting to
a 16bpp or 32bpp mode sprite does not require any translation tables, and
redirecting to a sprite that has the same mode and palette as the previous
destination that Tinct was used for causes a minimum overhead as the
translation tables are checked and cached values are used if possible.

Format of a sprite with 8-bit alpha channel
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The sprite format used by Tinct differs from those used by RISC OS Select,
and whilst facilities are supplied to convert sprites into the required format,
no facilities are provided to manipulate them.
 All sprites used by Tinct must be 32bpp, and cannot have a standard RISC OS
mask specified. The basic format of the sprite is shown below, with the
restrictions to the standard sprite format marked with an asterisk (*):

    [+0]    Offset to next sprite
    [+4]    Sprite name, up to 12 characters with trailing zeroes
    [+16]    Width in words - 1
    [+20]    Height in scan lines - 1
    [+24]     First bit used
    [+28]     Last bit used
    [+32]    Offset to sprite image
    [+36] *    Offset to sprite image (no mask allowed)
    [+40] *    Sprite type (must be 0x301680B5)

Whereas for normal sprites the sprite image would be a series of colour words
of the format RrGgBb00, alpha-blended sprites use the empty byte to specify
the alpha value, ie RrGgBbAa.
 The alpha values represent the blending level on a linear scale where 0x00
represents that the source pixel is totally transparent and 0xff that it is
totally opaque. It should be noted that as a standard 32bpp sprite (eg as
created with !Paint) will have the alpha channel set to 0x00 by default no
output will be visible when plotting as an alpha-blended sprite.

Error handling
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If an incorrect sprite is attempted to be used, Tinct currently always returns
error number 0x700 (SBadSpriteFile) rather than the specific cause of the
problem (eg. BadDPI, BadMSFlags or BadPixelDepth) as OS_SpriteOp would do.
There are several technical reasons for this behaviour, and future versions of
Tinct may return more descriptive errors depending on the cause.


SWIs provided
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Tinct provides four SWIs to plot sprites and one to convert sprites to their
32bpp equivalent. All values supplied to Tinct must be in OS units, and the
current OS clipping rectangle is used.
 The sprite pointers provided are equivalent to calling OS_SpriteOp with
bit 9 of the reason code set. To plot a sprite by name, the sprite should
first be found by using OS_SpriteOp with reason code 0x18 and using the
returned sprite address.

Tinct_PlotAlpha (0x57240)
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Plots an alpha-blended sprite at the specified coordinates.

->    R2    Sprite pointer
    R3    X coordinate
    R4    Y coordinate
    R7    Flag word


Tinct_PlotScaledAlpha (0x57241)
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Plots a scaled alpha-blended sprite at the specified coordinates.

->    R2    Sprite pointer
    R3    X coordinate
    R4    Y coordinate
    R5    Scaled sprite width
    R6    Scaled sprite height
    R7    Flag word


Tinct_Plot (0x57242)
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Plots a sprite at the specified coordinates with a constant 0xff value for
the alpha channel, ie without a mask.

->    R2    Sprite pointer
    R3    X coordinate
    R4    Y coordinate
    R7    Flag word


Tinct_PlotScaled (0x57243)
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Plots a scaled sprite at the specified coordinates with a constant 0xff value
for the alpha channel, ie without a mask.

->    R2    Sprite pointer
    R3    X coordinate
    R4    Y coordinate
    R5    Scaled sprite width
    R6    Scaled sprite height
    R7    Flag word


Tinct_ConvertSprite (0x57244)
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Converts a paletted sprite into its 32bpp equivalent. Sufficient memory must
have previously been allocated for the sprite (44 + width * height * 4).
 As sprites with 16bpp or 32bpp do not have palettes, conversion cannot be
performed on these variants. All sprites must be supplied with a full palette,
eg 8bpp must have 256 palette entries.

->    R2    Source sprite pointer
    R3    Destination sprite pointer


Tinct_AvailableFeatures (0x57245)
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Returns the features available to the caller by specifying bits in the flag
word. The features available are unique for each mode, although the current
version of Tinct supports the same subset of features for all modes.

->    R0    Feature to test for, or 0 for all features
<-    R0    Features available



Flag word
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All the SWIs provided by Tinct for plotting use a common flag word to
describe the manner in which the plot is performed. Each bit controls a
particular characteristic of the plotting:

    0  Forcibly read the screen base (only use if hardware scrolling)
    1  Use bi-linear filtering when scaling sprites
    2  Dither colours in 16bpp and below
    3  Perform error diffusion if bit 2 clear, invert dither pattern if set
    4  Horizontally fill the current graphics window with the sprite
    5  Vertically fill the current graphics window with the sprite
    6  Forcibly read the palette (only use if changing palette outside of
       the WIMP)
    7  Use OS_SpriteOp to perform final plotting (see note)
    8+ Reserved (must be 0) if bit 7 is clear, background colour to
       blend the alpha channel to otherwise

If a bit is set in the flag word that cannot be honoured by the current
version of Tinct then it is ignored. Tinct_AvailableFeatures can be used
to test in advance what flags will be honoured.

Bi-linear filtering
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Although bi-linear filtering is only relevant during scaled plotting, this
situation occurs when the EigFactors of the mode are not equal. As such, an
application should always set their preferred flags to ensure consistency. The
case of XEig * 2 = YEig (rectangular pixel modes) for even height sprites is a
special case and has optimised code implemented.
 There is an upper limit to the size of sprite that can be bi-linear filtered.
The checks that are currently made are:

    scaled_width / sprite_width < 256
    scaled_height / sprite_height < 256
    scaled_width * max(sprite_height, scaled_height) < 32,768

 It should be noted that as bi-linear filtering is performed as a pre-filter,
it carries a sizable overhead. However, as all scaling calculations are
performed during this filter, tiled plotting (bits 4 and 5) are affected by
a smaller margin (in certain cases a speed gain can be achieved).
 As bi-linear filtering is performed using a pre-filter, it can be used in
association with OS_SpriteOp rendering.

Error diffusion and dithering
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If both error diffusion and dithering are enabled then the image is plotted
using only dithering, but with the dither pattern inverted. This enables an
application to provide the user with what appears to be a higher quality image
by redrawing every frame with the flag toggled.
 There is a significant speed difference between dithering and error diffusion,
and Tinct does not support error diffusion in all colour depths. If error
diffusion is requested, but cannot be performed by Tinct then dithering with
an inverted pattern is used (as if bits 2 and 3 were set).
 There is an upper limit to the size of sprite that Tinct can perform error
diffusion on. This is currently set to a display width of 2047 pixels wide with
an unlimited height. Any attempt to use a higher resolution will result in
dithered rendering with an inverted pattern (ie bits 2 and 3 set).
 As error diffusion and dithering are implemented during the plot cycle, it is
not possible to use them in association with OS_SpriteOp rendering. However,
the bits should be set as future versions of Tinct may respect them for users
of RISC OS 3.1 where true colour sprites are not supported.

Sprite filling
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If filling is specified, then the supplied co-ordinate is the offset of the
pattern relative to (0, 0) used for the fill. For example, a 64x64 sprite that
is plotted with bits 4 and 5 set and a position of (32, 16) would fill the
current graphics window with multiple copies of the image stating with the
first image plotted at (-32, -48).
 The caller should not concern itself with the size of the image being tiled
as small images are internally optimised where possible to maximise the
plotting speed.

Rendering using OS_SpriteOp
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It can be useful to use Tinct to perform the rendering to using OS_SpriteOp.
There are two general situations where this may be useful:

    1) To output to a printer driver
    2) To allow hardware acceleraton (such as a ViewFinder card)

By using Tinct rather than a direct OS_SpriteOp call, it allows the caller to
retain certain features Tinct provides (such as sprite filling and a limited
version of the standard alpha blending) and allows the caller to have a common
plotting interface.
 When using this feature for alpha-blended sprites, the background colour
specified in the top 24-bits of the flag word is used for blending with any
pixels that are not transparent. This requires that Tinct requires a second
copy of the sprite in memory to modify which may present a significant overhead
in some situations. Plotting opaquely does not have any such overheads.
 Using OS_SpriteOp rendering does not currently work on RISC OS 3.1 or earlier
due to the lack of support for true colour sprites. Future versions of Tinct
may remove this restriction.

Contact details
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If you would like to report a problem relating to Tinct, provide feedback, or
request a licence for a commercial product, please use the details below:

Address: 5 Queens Close, East Markham, Newark, Nottinghamshire, NG22 0QY. UK
E-mail: [e-mail]
Website: www.tinct.net


Copyright and licence details
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Tinct is © copyright Richard Wilson, 2004.

Distribution and usage
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Unrestricted use of Tinct is hereby granted for any non-commercial product. Any
use as part of a commercial product requires written consent from the author.
No charge may be made relating to the distribution of this software, and this
file should be included in all copies of the software.
 Modified versions of this program may not be distributed without the authors
consent, nor may modified versions of the source code or relating files.